In the medical field, activity of human and animal brains has been scanned in various ways. Two of the more common ways of scanning brain activity are EEG and fMRI.
EEG technique involves measurements of electrical signals generated by the brain's neurons, via a multitude of electrodes placed on a patient's scalp. The neural signals are transmitted by wires to an EEG monitoring system that records the neural signals, and generates data about the signal variation in time, which data can be further analyzed and possibly also displayed. EEG enables high temporal resolution, in the order of milliseconds, and is therefore useful for detecting quick changes in the electrical activity of the brain. EEG, however, has undesirably low spatial resolution.
MRI is a noninvasive medical imaging technique used in radiology to visualize detailed internal structure and limited function of the body. In MRI, a magnetic field is used to align the nuclear magnetization of predetermined materials in fluids of a body. Radio frequency (RF) fields are used to systematically alter the alignment of this magnetization. This causes the materials to produce a rotating magnetic field detectable by a scanner. This signal can be manipulated/affected by additional magnetic fields to build up enough information to construct an image of the body. Different types of MRI generally used for brain scan are, for example, fMRI and magnetic resonance spectroscopy (MRS).
fMRI technique has been used since the 1990s to study the hemodynamic response (change in blood flow) related to neural activity in the brain. In fMRI, levels of deoxyhemoglobin in the brain are detected. Deoxyhemoglobin levels change in areas of the brain in response to changes in neural activity of the specific areas. Because deoxyhemoglobin is paramagnetic, when a patient's brain is subjected to a high magnetic field, deoxyhemoglobin aligns with the magnetic field. Radio frequency fields are then applied to the patient's brain, to rotate the aligned deoxyhemoglobin. As the paramagnetic deoxyhemoglobin rotates back and forth under the influence of the RF fields, a rotating magnetic field is produced, detected by an appropriate detector, and converted into an image of the brain. fMRI is characterized by high spatial resolution of measurements, as different levels of deoxyhemoglobin in specific regions of the brain indicate different levels of neural activity in those specific regions. fMRI is, however, characterized by relatively low temporal resolution, as blood flow response to neural activity peaks approximately 5 seconds after the firing of the neurons.
MRS technique utilizes the principles of MRI and focuses on receiving rotating magnetic fields produced by nuclei such as hydrogen, phosphorus, carbon, sodium, and fluorine, in order to detect biochemical materials of interest, such as, for example, choline containing compounds, creatine, inositol, glucose, N-acetyl aspartate, alanine, and lactate. Elevated concentrations of some of these biochemical materials may indicate the presence of a number of brain diseases, for example cancer, epilepsy, Alzheimer's Disease, Parkinson's disease, and Huntington's Chorea.
To combine the high temporal resolution of EEG and the high spatial resolution of fMRI, medical personnel have been increasingly performing simultaneous EEG and fMRI scans. Similarly, other kinds of MRI scans, such as MRS, may be performed in simultaneously with EEG measurements. This may be done, for example, for the purpose of saving time. The combination of EEG and fMRI scans, otherwise known as EEG/fMRI, however has brought about new problems associated with the fact that EEG measurements are affected by noise artifacts, when performed in the presence of a magnetic field. Therefore, noise artifacts have appeared in EEG measurements performed in conjunction with fMRI scans. Similarly, when using MRS measurements simultaneously with EEG measurements, EEG scans are also affected by noise artifacts caused by the presence of a magnetic field.
Noise can also be introduced into the EEG signals during EEG recording within an MRI scanner. Specifically, noise may be introduced by motion within the MRI environment during the recording of the EEG signals. This type of noise is called a motion artifact. Motion artifacts may be, for example, associated with a ballistocardiogram motion, and/or a movement of the patient during the EEG recording, and/or a movement of EEG equipment during the recording. The amplitude of the noise may be approximately of the same magnitude as the EEG signal.
Some motion artifacts are present as a direct result of an electromagnetic induction in the magnetic field. In EEG scans, as motion causes EEG wires to move within the MRI scanner's magnetic field, the size of the loops created by two wires connected to the electrodes varies, affecting the magnetic flux through the loops, and therefore introducing an induced current through the loops.
Another technique used in conjunction with EEG is Transcranial Magnetic Stimulation (TMS). In TMS, a changing magnetic field is used to induce weak electric currents in the brain; this can cause activity in specific or general parts of the brain, allowing the functioning and interconnections of the brain to be studied. Such activity is generally detected via an EEG apparatus. A variant of TMS, repetitive transcranial magnetic stimulation (rTMS), has been tested as a treatment tool for various neurological conditions such as migraines, strokes, Parkinson's disease, dystonia, tinnitus, depression and auditory hallucinations.
EEG recordings performed during TMS may be affected by a motion artifact, as explained above. Further noise affecting EEG measurements in a TMS environment includes a gradient artifact produced by interference in the electrical loops due to the changes in the magnetic field. This gradient artifact present in EEG measurements during TMS is also known as a magnetic pulse artifact. More specifically, the alternating magnetic field affects magnetic flux through the loops and induces currents, thus creating a gradient artifact in the EEG measurements.
Techniques have been devised to decrease the motion artifact and/or the gradient artifact. According to one system used in the art, wires are tightly held together in bundles essentially parallel to each other. The wires may be twisted together to form the bundles. In this manner, a size of loops formed by wires belonging to the same bundle is decreased, thereby decreasing the motion artifact in EEG readings. Reduction of the loop size also reduces current induced by an alternating magnetic field. This approach enables bipolar EEG measurements with a decreased presence of motion artifacts and/or gradient artifacts.
Another known technique of the kind specified is described for example in US Patent Publication No. 2008/0306397 by Bonmassar et al. which discloses methods, systems and arrangements for obtaining EEG signals from a patient e.g., during a concurrent EEG/fMRI examination of the patient. The methods, systems and arrangements include a cap made of conductive inks with sensor positions for attaching a plurality of sensors to the patient's head. The sensors can include electrodes as well as motion sensors for improving EEG signal quality and MRI image quality in the presence of motion noise and other artifacts within the MRI environment. The electrodes may be composed of conductive inks, and can be used in high magnetic fields due to a weak interaction with the RF fields generated by the fMRI scanner.
Motion artifacts may generally be classified in two categories: Balisto-Cardio-Gram (BCG) and Non-Balisto-Cardio-Gram (NBCG). BCG artifacts are related to scalp pulsation and head movements caused by heart beats. NBCG artifacts are generated by movements which are unrelated to heart beats. Several techniques have been proposed for suppression of BCG artifacts. One such technique is based on subtraction of averaged BCG artifact from the EEG data after calculation of averaged template from subsequent BCG artifacts (Allen P. J, Polizzi G, Krakow K et al, 1998, Identification of EEG events in the MRI scanner: the problem of pulse artifact and method for its subtraction. Neuroimage 8 (3), 229-239). Another similar technique is based on weighted average (Goldman R I, Stern J M, Engel Jr. J, Cohen M S, 2002. Acquiring simultaneous EEG and functional MRI. J Clin Neurophysiol. 111, 1974-1980). A further technique, known as optimal basis function set method (Niazy R K, Beckmann G D, Iannetti J M et al, 2005, NeuroImage 28, 720-737), employs principle component analysis (PCA) to decompose the EEG data into several functions which are fitted to every heart beat artifact. Yet a further technique uses a piezoelectric sensor on skin above temporal artery (Bonmassar G, Purdion P L, Jaaskelainen I P et al, 2002. Motion and balistocardiogram artifact removal for interleaved recording of EEG and EPs during MRI. Neuroimage 16(4), 1127-1141), in order to capture the temporal characteristics of the arterial blood pulse.